Myositis, an autoimmune muscle disease, likely develops as a result of environmental exposures in genetically susceptible persons. Preliminary data suggest a trend for an increasing incidence in myositis in military personnel over the last decade for unknown reasons. Although a few environmental exposures have been preliminarily associated with myositis in the civilian population, these have not been confirmed. In addition, no study has assessed exposures that might result in the development of myositis in military personnel. Military personnel experience a number of intense, unique exposures, often over a relatively short interval, which include different stresses, novel vaccines, distinct occupational exposures, battlefield injuries and unique chemicals during field deployment, that differ from those exposures in non-military populations. Therefore, we propose a protocol that consists of three complementary approaches to attempt to determine the environmental factors associated with the development of myositis in active duty military personnel and an initial understanding of the possible mechanisms involved. In the first approach, we will assess risk factors in a case-control study of 300 patients who developed myositis while on active duty by comparing them to 1500 active duty military personnel (randomly selected, but matched 5:1 by gender, race, and age and military service within 10 years) who have not been diagnosed with an autoimmune disease or chronic muscle disease. For this first approach, we will analyze existing military databases for information on medications, vaccines, infections, co-existing medical conditions, military occupations, deployments, and worldwide active duty locations. In the second case-control approach, we will attempt to define environmental factors associated with the development of myositis that developed in military personnel (n=150) by comparing them to 150 similarly matched military personnel who have not been diagnosed with an autoimmune disease or chronic muscle disease. This second approach will differ from the first approach, in that subjects will be prospectively enrolled and assessed during a single clinic visit to confirm diagnoses and examine patient questionnaires on focused environmental exposures, including those not captured in the military databases. A third laboratory approach will identify, in an exploratory study, the global DNA methylation epigenetic changes, microRNA and mRNA profiles in peripheral blood and muscle tissues from 18 subjects (six PM and six DM compared to six non-myositis controls enrolled in the second approach) and assess the effects of selected environmental exposures on these parameters. These complementary approaches should enhance the understanding of environmental factors and possible mechanisms associated with the development of myositis in the military, and provide insights into environmental risk factors that may also be relevant to the development of myositis in non-military populations.